If history is true, the devil is in the details.
From what I understand (and this is only me trying to put things in layman's terms).

If you simply upgrade the current system to W10, you have 30 days until the OS starts writing over files that make it possible to revert back. At this point you are stuck with W10 without doing a new install/repair.

If you make a clone, and then upgrade the original, you will likely have more than 30 days to revert back to the original drive.

If you try to load your older clone after 30 days, you'll likely have to re-authenticate it. This will be a pain but doable. By coincidence, 30 days is how long you can run an nu-authenticated OS on Windows currently.

If you decide to revert back to W7, you'd have to make a phone call to re-authenticate the license for W7.

Why do I think this?
Scenario1: you upgrade to W10, your system has an issue and totally crashes, and your OEM restore disk is a W7 restore/repair. Do you think they will abandon supporting these? They may make it a bit of a pain, but I don't think they would abandon re-authenticating a W7 OEM restore ..

Just my opinion, based on previous experience.
They don't want the license used on more than one machine. Even if the machine does have a different MB, it can be authenticated, but only for one machine.

Scenario2: Dual-boot; I do not know if they will allow 2 OS to share a license on the same machine. I'm sure they'd frown on it, but I can't answer.
History says this was possible, but whether W10 changes that will be interesting.

I do not work here, but the benefits are still awesome
Make your sound your own!